An MRI apparatus measures an NMR signal generated by nuclear spins which form an object, especially, human body tissues, and generates morphology or functions of the head, the abdomen, the limbs, or the like as a two-dimensional or a three-dimensional image. During imaging, radio frequency (RF) pulses for exciting an object, and gradient magnetic field pulses having a phase encoding direction and a frequency encoding direction are applied according to a predetermined imaging pulse sequence. Consequently, an NMR signal emitted by the object is subject to phase encoding and frequency encoding so as to be measured as time-series data. The measured NMR signal is subject to two-dimensional or three-dimensional Fourier transform and is thus reconstructed into an image.
In the MRI apparatus, in order to minimize an influence such as a temperature increase on an object, exerted by the RF pulses, it is necessary to reduce a specific absorption rate (SAR) (an absorption amount of RF per unit mass) to a threshold value or less according to a regulation of the International Electro technical Commission (IEC) (PTL 1). An expression for calculating an SAR is given by the IEC.
In addition, as disclosed in NPL 1, it is known that a Q value of an RF applying device (an irradiation coil) is necessary in order to accurately measure or predict an SAR. The Q value is generally known as a parameter indicating the sharpness of resonance, but the Q value of the RF irradiation coil of the MRI apparatus depends on an internal resistance of an object irradiated with an RF pulse. For this reason, in the related art, a Q value is predicted and used by using an actually measured value of the RF irradiation coil or a region, measured in the past.